In order to produce halftone images with ink-jet printers, dots of varying diameter must be created on the surface being printed. The state of the art offers various methods of achieving this object.
WO 87/03363 describes a method and an apparatus for creating halftone images with eight shades in which the ink droplets produced vary according to a binary ratio of their weights. The print head is constructed in such a way that for each color of ink there are orifies whose diameters vary according to the ratio of, say, 1:2:4. The corresponding heater elements vary by approximately the same ratio. The orifices are controlled in such a way that the combination used ejects the required mass of ink to produce the size of dot desired on the surface being printed. In order to produce a halftone image, each row of orifices requires a further two orifices in order to achieve the eight shades referred to above. The number of heater elements also increases correspondingly.
EP-A-0 373 894 describes a method and an apparatus for creating halftone images. This involves passing two consecutive current pulses through a heater element in a print head. The first pulse is used to heat the ink, while the second is used to create a vapor bubble which then forces an ink droplet out of the ejection orifice. The degree by which the ink is preheated determines the mass of ink ejected by the second pulse. A similar method is described in EP-A-0 354 982. In the latter case, the initial pulses used to heat the ink are generated in various different ways. This might involve a single pulse of long duration or a number of separate short pulses. The amount of energy transmitted to the ink by the pulses determines the mass of the ink droplet.
European patent application EP-A-0 372 097 describes an apparatus for creating ink droplets of varying sizes in an ink-jet printer according to the thermal converter principle. This involves matching the control pulses to the geometry of the thermal converter. The thermal converters are divided into a number of heater sections (W1 . . . W3) of varying width (bi), thickness (di) or material ((I, ai). The heterogeneity of the heater elements means that certain heater element sections heat up faster than others. Those sections which heat up more slowly are used to heat the ink while those which heat up quickly are used to produce vapor bubbles.